The invention relates generally to transmitting and receiving data in a telecommunication system and, more particularly, to a system and method for coding and interleaving data onto separate channels for transmission in a multicarrier system.
Generally, a wireless telecommunication system serves a defined area by dividing the area into cells. Each cell is served by a single base station, or cell site, and each cell site is connected to a message switching center (xe2x80x9cMSCxe2x80x9d) via appropriate hardware links. A mobile unit is connected to the MSC by establishing a link with a nearby cell site using one or more radio frequency (xe2x80x9cRFxe2x80x9d) channels, or carriers.
For various reasons, higher data rates are desirable in the RF link between the mobile unit and the cell site. For example, in a code division multiple access (xe2x80x9cCDMAxe2x80x9d) system, data rates are somewhat restricted due to current standards that require a system bandwidth of 1.25 Megahertz (MHz). However, future requirements are expanding to support a system bandwidth of nxc3x971.25 MHz, where n is an integer, thereby supporting backwards compatibility to the traditional 1.25 MHz bandwidth.
Once a system bandwidth of nxc3x971.25 MHz is chosen, there are two conventional methods by which the data may be transmitted over the chosen bandwidth. One method, referred to as direct spread, encodes the data using a convolutional code, interleaves the resulting coded bit stream, encodes the interleaved symbols using a Walsh code unique to the user, further encodes the encoded symbols using a pseudo-noise (PN) code, and then transmits the resulting symbols, known as chips, on the entire nxc3x971.25 MHz bandwidth. Using this method, the actual symbol rate on the RF carrier is nxc3x971.25 million chips per second. Another method, referred to as multicarrier, splits the encoded data into n streams, encodes each stream using a Walsh and a PN code, and then transmits the resulting chips of each stream on a separate 1.25 MHz carrier. A CDMA compatibility standard for cellular mobile telecommunications systems, including greater detail in the above described methods, is described in MOBILE STATION-BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE WIDEBAND SPREAD SPECTRUM CELLULAR SYSTEM, TIA/EIA/IS-95 (July 1993).
The multicarrier method described above does not adequately handle error conditions that occur over the separate carriers. For example, if several bit errors occur in sequential order, even the existence of error correcting code in the bit stream does not prevent certain data bits from being lost. To alleviate this problem, the multicarrier method interleaves, or shuffles, the order of the data bits separately on each carrier before transmission and then de-interleaves the data bits at the receiving end. This solution, however, has several deficiencies. For one, shuffling circuits are often used for each carrier, thereby requiring extra circuitry. Also, the shuffling does not take into account that certain carrier frequencies may be more robust at any particular time, and therefore subject to less errors than the other carrier frequencies.
Therefore, what is needed is a system and method that codes and interleaves data onto separate carriers for transmission in a multicarrier system.
Furthermore, what is needed is a system and method that does not require individual interleaving circuitry for each carrier of the multicarrier system.
Furthermore, what is needed is a system and method that utilizes a different carrier for consecutive bits of the error correcting code, thereby providing a versatile system that is less prone to errors.
The foregoing problems are solved and a technical advance is achieved by a system and method that codes and interleaves data onto separate channels for transmission in a telecommunication system. In one embodiment, the multicarrier telecommunication system has an input terminal for receiving a plurality of user data bits. The system may add error-correcting bits to the user data bits. The system then arranges and stores the combination of error-correcting bits, collectively coded symbols, in a two dimensional matrix. The system arranges the symbols by writing them into the matrix according to columns, from left to right. The system then retrieves the symbols from the matrix according to rows, from top to bottom and transmits the retrieved symbols on different carrier frequencies such that consecutive symbols are each transmitted on a different carrier frequency.
As a result, the system reduces the effect of errors resulting from any single carrier. Furthermore, the arranging of the symbols is performed at a central location, thereby obviating a need for each carrier to have its own shuffling device.